Fuel cells are well known and are commonly used to produce electrical current from reducing fluid fuel and oxygen containing oxidant reactant streams, to power various types of electrical apparatus. Known solid oxide fuel cells generate both electricity and heat by electrochemically combining a fluid reducing fuel and an oxidant across an ion conducting electrolyte. In a typical oxide fuel cell, the electrolyte is an ion conductive ceramic membrane sandwiched between an oxygen electrode (cathode) and a fuel electrode (anode). Molecular oxygen, such as from the atmosphere, reacts with electrons at the cathode electrode to form oxygen ions, which are conducted through the ceramic membrane electrolyte to the anode electrode. The oxygen ions combine with a reducing fuel such as a mixture of hydrogen and carbon monoxide to form water and carbon dioxide while producing heat and releasing electrons to flow from the anode electrode through an electrical circuit to return to the cathode electrode.
Solid oxide fuel cells have many benefits and many limitations. For example, normal operating temperatures are very high, often in excess of 700° C., which favors stationary power plants operating in a near steady-state mode to minimize deleterious effects of thermal cycling as the fuel cell is started up and shut down. However, because of the high operating temperatures, it is known that solid oxide fuel cells may use a variety of reducing fuels and some of these fuels do not have to be reformed into pure hydrogen gas prior to entering the fuel cell. Additionally, solid oxide fuel cells are known to have long term operating stability and relatively low emissions of undesirable exhaust gases.
Recent developments in improving the efficiency of solid oxide fuel cells have included utilizing an oxidizable molten metal as an anode electrode. U.S. Pat. No. 7,943,271 that issued on May 17, 2011 to Tao et al. and assigned to CellTech Power, LLC shows an anode including a molten tin alloy that has resulted in significant potential benefits. These benefits include system simplification requiring no fuel reformer to produce a synthesized gas fuel (“syngas”) including varying amounts of carbon monoxide and hydrogen. In use of the molten tin anode, the tin is oxidized to tin oxide by the oxygen ions passing through the cell electrolyte, which releases electrons. The tin oxide is then reduced by the reducing fuel back to tin.
While use of a molten tin anode has potential for efficient fuel usage, primary drawbacks include difficulties of managing a highly conductive, extremely hot molten metal within a fuel cell, and in particular within a stack of fuel cells that are generally layered upon each other to form a fuel cell stack. In order to build voltage in a fuel cell stack, groups of adjacent fuel cells are typically arranged electrically in series. A hot and highly conductive liquid anode electrode requires extremely complex control of movement of the molten metal anode to avoid short circuits between adjacent cells. To minimize such problems, it is known to utilize solid oxide fuel cells in a tubular, non-planar arrangement, wherein fuel cells of a fuel cell stack are wired together to build voltage, externally (i.e., by means of wiring that is external to the individual cells), and electrical connections are made at near room temperature, which is a complex and costly arrangement. Such a complex tubular arrangement in a fuel cell with a molten tin alloy anode is shown in U.S. Pat. No. 7,943,270 that also issued on May 17, 2011 to Blake et al. and that is also assigned to CellTech Power, LLC.
A further limitation of utilizing molten tin as an anode is that solid tin oxide (SnO2) forms within the molten tin and tends to block a surface of the electrolyte thereby degrading cell performance. Tin oxide melts at 1,630° C., thus requiring the molten tin anode to be maintained at an extremely hot temperature.
Consequently, there is a need for a solid oxide fuel cell that overcomes the limitations of known solid oxide fuel cells.